Generally speaking, high intensity discharge (HID) arc tubes need help starting an arc between electrodes at opposed ends because typical operating voltage levels are not high enough to initiate an arc. Historically the starting method has involved supplying high voltage spikes to the lamp lead-in wires during lamp startup. The spikes or pulses are generally provided by “ignitor” circuitry/components that are usually outside the lamp jacket, typically as part of the ballast, therefor the lamp supply wiring and lamp socket connectors must be able to deliver the maximum pulse voltage without arcing. FIGS. 1A (magnetic ballast with ignitor) and 1B (electronic/EL ballast with high frequency ignition circuitry) illustrate starting pulse waveforms typical of present ballasts. The EL ballasts supply operating power, and superimpose high frequency (e.g., about 200 kHz), high peak voltage ignition pulses that are repeated (e.g., every second) until arc ignition is achieved.
Adjusting HID lamp design factors to increase lamp performance can make the arc harder to start, and it has become very difficult to economically and safely supply starting voltage spikes any higher than they are now (e.g., ˜3 kV superimposed on the steady state lamp supply voltage).
It is known that ignition waveform requirements such as spike/pulse peak voltage can be decreased by using starting aids such as antennas and/or UV ionization sources. Conversely this means that starting aids can enable higher performance lamp designs that start more reliably at existing peak starting voltage. This is illustrated in FIG. 2, where starting reliability for different types of antenna starting aids are compared to starting without an antenna starting aid.
For example, in HID lamps the radiant output in desired spectrum bands (e.g., PAR radiation for horticultural lamps) can be improved in various ways by increasing the cold fill pressure of xenon gas, and/or by modifying other design factors such as the arc gap, etc., however such design changes raise the starting voltage requirement. High performance HID lamps have been pushing the envelope until they are limited by starting aid capabilities, therefor an improved starting aid is needed.
A known starting aid for arc tubes made with PCA (PolyCrystalline Alumina) such as in HPS (high pressure sodium) and CMH (ceramic metal halide) lamps, is called an “antenna”, which is a wire-like conductor extending longitudinally along the arc tube (usually outside of it) between the two electrodes. It is sufficient for the antenna to bridge the arc gap with antenna ends located radially outward (above or outside) of each of the two electrodes, and may include conductive rings around the tube that are electrically connected to each antenna end. As such, this is called a “passive antenna” because it is electrically floating and can be inductively charged by the AC voltage being applied across the nearby electrodes. A preferred implementation of this is where the conductors are applied (“printed”) on the outside arc tube wall and bonded by sintering.
As shown in FIG. 2, a much more effective antenna is an “active antenna” where the antenna is electrically connected to at least one of the electrode lead wires. The problem with this is that having a live wire (always-charged conductor) near the arc tube wall causes metal ion migration away from the arc (e.g., “sodium loss”) which collects on the inner arc tube wall, leading to deterioration of the wall. The ions may then migrate through the wall, which in worst case scenarios creates a conductive path to the outside of the arc tube which bypasses the internal arc gap to cause a catastrophic failure.
Thus recent design efforts have been directed to development of a starting aid that provides the benefits of an active antenna, but avoids its problems, thus allowing design changes to further improve performance without causing starting problems. For convenience in the present disclosure, where the improved starting aid is based on an antenna, we may call it a “semi-active antenna”.
U.S. Pat. No. 8,456,087 to Steere et al. and assigned to Koninklijke Philips Electronics N.V. (the Philips '087 patent) discloses a “High-Pressure Sodium Vapor Discharge Lamp With Hybrid Antenna” comprising an HID lamp arctube wherein a printed antenna is indirectly coupled to an electrode lead to produce a “hybrid antenna”. This is described with reference to drawings such as their FIG. 4D that is reproduced herein as FIG. 3 (Prior Art). Summarizing the Philips '087 disclosure: an exemplary HPS (HID) lamp 200 has an antenna lead 212 that is a conductor extending from the antenna main part (206) to a point close to the electrode feedthrough 208 in a lead-in hole 223, where the antenna lead is separated a distance PD from the electrode lead (feedthrough) by sealing frit 210, which may be electrically conductive with a resistance, or may be substantially nonconductive. The antenna lead 212 may extend 213 inward on a portion of the inner wall 240 of the lead-in hole 223. The separation distance PD may be in the range of 20-100 microns, resulting in a resistive and/or capacitive coupling between the antenna 206 and one or both electrodes 236.
Thus the “hybrid antenna” appears to be Philips' implementation of an antenna wherein the printed antenna is indirectly (capacitively and/or resistively) coupled to an electrode lead by means of an electrical conductor that is connected to, and extends from, the printed antenna to the end of the arctube and continues around the end down toward the electrode lead.
It is an objective of the work disclosed herein to develop a novel form of antenna that enables improvement of HID arc tube performance while avoiding starting problems and/or other negative effects like those described above.